Lactation and especially milk, which is the product of that unique mammalian process, are routinely encountered within our daily lives. Nevertheless, they often are poorly understood by many, even including many who are engaged in the business of producing milk. The overall course goal is to introduce fundamental concepts that form the basis for understanding the biology of lactation, the biology of the mammary gland, and the products of that important physiological process.
As a learner in this course, you will be provided with a series of easily understood presentations that collectively will help you build a foundation for greater understanding of lactation. You will be able to engage with other learners so that you can extend your learning beyond the video presentations. Ultimately, you will be able to construct your own mental model for understanding the wide range of topics that relate to the biology of lactation. Upon completion of the course, you will be prepared to expand your knowledge and understanding of lactation from other sources and experiences as you pursue your individual interests.
Before you start the course, I suggest that you identify a question or several questions about lactation that you already have on your mind. This could be from your own experiences, something you read about or saw, or something you have wondered about. Write down your question(s) and use that to help you decide how to engage with the content of this course. You might engage with the modules in the order they are presented, or start with a module that is of particular interest to you, or pick and choose modules in any order. I encourage you to engage in all of the types of learning activities that this course has to offer, including but not limited to, the discussion forums, quizzes, peer-review assignments, and concept maps and other learning aids.

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Module 6: Lactation

In this module, we explore the many facets of lactation as a complex physiological process. We will find that removal of the milk in the gland is critical to continued production of that fluid. The interactions between milk removal, the systemic factors stimulating the gland to produce milk, such as several hormones, and the local factors that inhibit further milk secretion are examined. Some of the effects of milk removal, as well as the absence of milk removal, are discussed in this series of videos. The responses of the mammary gland to milk removal provide a means of understanding the impact of management practices on lactation.

Ministrado por

Dr. Walter Hurley, PhD

Professor Emeritus

Transcrição

We're going to continue talking about lactation and milk removal. And today in this video we're going to talk a little bit about frequency of milk removal. And some of the implications of that, especially with regard to the dairy cow. But just to kind of remember what's going on here, we have those systemic factors. Prolactin release is one of those, stimulating the mammary gland to start refilling and making milk. On the other end, on the other kind of part of it, more of the local part in the mammary gland, we have the feedback inhibitor of lactation, which is shutting down further milk secretion. Remove the milk, again we start the cycle back over again. So this removal of milk, getting rid of the milk, getting the feedback inhibitor of lactation out of the mammary gland. Stimulating that release of prolactin, and as we've seen, some other hormones as well, gets the gland going again to refill. And that's really what's keeping lactation going. That's part of maintenance of lactation. So now we want to examine this a little bit closer. And again, we're going to focus on the dairy cow in this particular video. But some of the underlying concepts will be valid for many other species as well. And we're going to focus on, again, frequency and set that up for you. So let's take a look at the first slide here. So, on this axis, we have milk secretion rate. So that would be milliliters per minute or whatever the case may be. And down here, hours after last milking. And this is kind of a general shape of a curve. Something very similar to this that we would see in the rate of milk secretion in the mammary gland dropping off after the last milking, which is 0 time here. And when the gland is filling up you're getting accumulation of feedback inhibiting lactation. The gland is trying to keep making milk, but again you're getting this negative feedback. You kind of lost that, or no longer have that prolactin stimulus there. So, again, secretion rate starts dropping off at an ever-increasing rate. And in fact, what we'll see here is roughly about 35, 36 hours, it's pretty well stopped in this particular species. Again, this is the dairy cow. What we'll find in some other species later on is it probably stops a bit earlier than that. The dairy cow is actually pretty adaptable. Keeps the secretion rate going for perhaps a longer period of time than many other species. How does this fit into managing dairy cows, and how we milk them, and how often we milk them? Lets go to the next slide. If we milk them on a 12 hour interval, which you can kind of see here. I've kind of drawn this out. The ideal, of course, would be if milk secretion rate never dropped off. If she kept producing milk at a constant rate, of course it would go along like this. But it does drop off. And so this increment we have right here represents the loss of milk that's occurring during this time where the milk secretion rate is dropping off between the last milking and then the next 12 hour milking. On the other hand if we milk them three times a day it's at 8 hour intervals, something like this. You can see that the decrease here is smaller so we would lose a little bit less. In fact if we go to a three times a day milking, typically they talk in terms of like say a 25% increase of milk yield, roughly something like that. And part of that, not all of it, but part of it is because of the difference in terms of, compared to two types of data at any rate, what you're losing here in terms of where they are in the secretion rate curve. And so again, there's relatively less milk lower than the optimal here at 8 hours versus 12 hours. So that kind of sets us up for how we think about milking two times a day versus three times a day. Let's go to the next slide. This is a curve which represents, what would it look like? What would the lactation curve look like? And they might perhaps start out, well, at the same point because they're not being milked prior to calving. Again, you can see the increment here of increase is higher. Higher peak is attained at three times a day milking, and then comes down pretty close to similar rate of decline. But obviously the area under the curve, or the yellow line here, the three times a day milking is going to be significantly greater than two times a day milking. And again, part of that gets back to what I just showed you in terms of what's going on in the mammary gland. Part of it is due to generally better management. Cows are seen three times a day here versus two times a day. They're probably fed and managed a little bit differently. So that's part of why this is up here. But again, part of that is because we're taking better advantage of the physiological process before that secretion rate really starts dropping off. Let's go to the next slide. So what we see here is that we get, typically, at peak lactation, milking three times a day, about a 10% increase in milk production compared to milking twice a day. So again, 12 hour intervals here, 8 hour intervals here, during a 24 hour period. And then out here later in lactation, the very end of lactation, it can be as much as a 40% increase. So again, they're not exactly parallel but they're coming down in a similar manner in this declining phase of lactation.